1. Field of the Invention
The present invention relates to a solid state imaging device.
2. Description of the Related Art
Heretofore, in an imaging camera such as a video camera or a digital still camera, a solid state imaging device is widely used. An exit pupil which is determined by a diaphragm of the lens used in an imaging camera does exist. In an imaging camera, an exit pupil distance which is a distance from the lens focal point to the exit pupil is finite, so that the principal ray which enters the solid state imaging device is inclined as the position is made away from the center of the optical system to the periphery thereof and the incident angle becomes larger.
FIG. 9 shows one example of a solid state imaging device. In this CCD solid state imaging device 1, a plurality of sensor portions 2 by means of photodiode are laid out and formed in a matrix form in an imaging region on a silicon semiconductor substrate 1 and it is constituted such that a vertical transfer register 3 of a CCD structure is formed at each sensor portion row. The vertical transfer register 3 is formed by arranging is formed by laying out a plurality of transfer electrodes 6 through a gate insulation film 5 on a transfer channel 4 in a transfer direction. A light shielding film 9 is formed through an insulation film 8 on the vertical transfer register 3 so as to coat thereof. Additionally, an in-layer lens (concave lens for this example) 12 is formed by a reflow film (insulation film) 10 and a flat formed film (insulation film) 11 which have different refraction indexes each other corresponding to each sensor portion 2. Further, an on-chip micro lens 15 is formed over the in-layer lens 12 through a color filter 13 and flat formed film 14 corresponding to each sensor portion 2.
In a solid state imaging device 1 of such a constitution, if the laid out pitches of the sensor portion 2 and the on-chip micro lens 15 are equal to each other, that is, if the on-chip micro lens 15 exists just over the sensor portion 2, the incident light L which enters the on-chip micro lens 15 at the periphery of the imaging region (see FIG. 9) is not focused to the center of the sensor portion 2 and it is shaded by the light shielding film 9 formed adjacent to the sensor portion 2 such that a sensitivity deterioration is caused. This becomes a cause of a sensitivity irregularity of a picture screen called as shading.
As a technology for correcting this shading, a method is generally used where the laid out pitch of the on-chip micro lens 15 is made smaller than the laid out pitch of the sensor portion 2 as shown in the patent reference 1. To be more concrete, as shown in FIG. 10, it is realized by performing a reduction magnification on the laid out pitch of the on-chip micro lens 15 centering the optical center of the solid state imaging device (center of the imaging region 17 in FIG. 10). With respect to the reduction magnification in this case, the laid out pitch of the on-chip micro lens is reduced by an equal magnification all over the imaging region. In this manner exit pupil correction is performed and as shown in FIG. 11, the incident light L which enters the on-chip micro lens 15 is made focused to the sensor portion 2 even at the periphery of the imaging region.
Also, a solid state imaging device has come to be installed in a mobile apparatus such as a mobile telephone and a personal information mobile terminal and an optical system where the exit pupil distance is very short has been used for miniaturization. For this reason, the light incident angle to the periphery pixels becomes moreover larger and the technology of the exit pupil correction for the on-chip micro lens has becomes more and more important.
In recent years, in addition to the conventionally used CCD image sensor for a solid state imaging device, a CMOS image sensor has come to be installed in a video camera, a digital still camera and a mobile apparatus for the reasons such as “accumulated manufacturing technology of the MOS structure can be used” and “the power consumption is low”. In this CMOS image sensor, it is necessary to arrange wiring layers of such as aluminum three dimensionally in the pixels for the reason such that each pixel has several transistors. In a CMOS image sensor, since there exists wirings of layers equal to or more than 2 as described, it is general to perform the light shielding between the pixels by the wiring of the uppermost layer for preventing the color mixture and the like. Further, it is necessary to arrange the readout gate which is the signal line within each pixel in order to read out the electric charge for every pixel.
FIG. 4 shows a conceptual constitution of an imaging region in a CMOS image sensor. A CMOS image sensor 21 is constituted by providing an imaging region where a plurality of sensor portions 23 by means of photodiodes are formed in a matrix form on a semiconductor substrate 22, one pixel is formed between this sensor portion 23 and a plurality of MOS transistors, an insulation film 24 between wiring layers of multiple layers which correspond to wiring layers 251, 252 and 253 of a first layer, a second layer and a third layer in this example are formed through an interlayer insulation film 24, and further, an on-chip micro lens 28 corresponding to a color filter 27 and rectangular sensor portion 23 thereon is formed through a flat formed layer 26. According to this CMOS image sensor 21, for example, the vertical signal line corresponds to a wiring layer 251 of the first layer; the horizontal reset line, the vertical readout line and vertical selection line correspond to a wiring layer 252 of the second layer; and the power supply line corresponds to a wiring layer 253 of the third layer. In this CMOS image sensor 21, the exit pupil correction of the on-chip micro lens 28 is also performed similarly as in FIG. 10.
Here, the form of the sensor portion and the opening shape of the light shielding film respectively will not become symmetrical in the vertical direction or the horizontal direction owing to the arrangement of the readout gates and the wiring layers. In this case, if an exit pupil correction for the on-chip micro lens is performed according a conventional manner, a constant correction is performed with respect to the distance from the optical center of the imaging device, so that places where sensitivity is especially deteriorated resulted from the sensor portion shape or the opening shape of the light shielding film cannot be avoided from existing.                [Patent Reference 1]        JP Laid-open Publication No. 01-213079        
Now, for example, in case when the opening of the light shielding film 32 is formed by a wiring layer 253 a of the uppermost layer and a CMOS image sensor has a sensor portion 23 of a shape where the left lower corner portion of a square shape is shielded obliquely as shown in FIG. 5, the sensitivity of each place becomes as shown in FIG. 5 when an optimum exit pupil correction is performed at the left upper and lower positions <2>, <3> and at the right lower corner position <4> on the imaging region (picture screen) 31 (see FIG. 6). More specifically, the sensitivity deterioration at the right upper corner position <1> of the picture screen 31 is remarkable.
Since a light L1 enters the position <1> from the direction which is obliquely light-shielded at the opening of the light shielding film, the light moreover increases, because it is shaded by the oblique light shielding portion 32 shown as a bold line portion in FIG. 5 when the exit pupil correction of the on-chip micro lens 28 is performed. Conversely, in case when the exit pupil correction amount of the on-chip micro lens 15 is optimized at the position <1>, it can be easily imagined that the sensitivity deterioration at the positions <2>, <3> and <4> are remarkable. When it is installed in an optical system in which the exit pupil distance is short such as for a mobile apparatus, the phenomenon appears more notably, so that the problem is serious.
According to another example, in a case of a CMOS image sensor having a sensor portion 23 of a shape where the left lower corner side is light-shielded obliquely and vertically as shown in FIG. 7, the sensitivity of each position when an optimum exit pupil correction is performed at the right upper and lower corner positions <1>, <4> and at the left upper corner position <2> on the imaging region (picture screen) 31 (see FIG. 8) becomes as shown in FIG. 8. More specifically, the sensitivity deterioration is remarkable at the position <3> of the picture screen 31.
Since the focusing position for the position <3> is deviated to a portion 32 which does not function as the sensor portion 23 of FIG. 7, a shaded light will occur. Conversely, when the exit pupil correction amount of the on-chip micro lens 28 is made optimized at the position <3>, it is easy to imagine that the sensitivity deterioration becomes remarkable at the position <1>, <2> and <4>.
For such shading, correction cannot be achieved by a conventional exit pupil correction of the on-chip micro lens any more and it happens that a remarkably dark place does exist in a picture screen. Further, the shading amount is not constant with respect to the distance from the center of the picture screen, it is difficult to apply a shading correction technology other than the exit pupil correction of the on-chip micro lens such as a technology in which a shading correction is performed by making the periphery of the picture screen bright electrically.
As mentioned above, a technology in order to propose a solid state imaging device where non-uniform shading which is resulted from the asymmetrical shape of the sensor portion and the shape of the opening of the light shielding film is corrected with respect to the distance from the center of the picture screen, the shading amount is made to be uniform with respect to the center of the picture screen and at the same time the sensitivity irregularity is reduced as a result so as to improve its sensitivity has been desired.